Neighboring Volcanoes:

There are six major volcanoes in Colombia, the closest major one to Nevado del Ruiz being Tolimo. In between the two major volcanoes, both located southwest of Bogota, are two volcanoes, Santa Isabel and Quindio. Each of these volcanoes are a stratovolcano similar to Nevado del Ruiz, and are located in a region called Parque Nacional Los Nevados.
Below is a map from USGS showing the locations of the six major volcanoes in Colombia.